Optical coherence tomography (OCT) is an interferometric imaging technique with widespread applications in ophthalmology, cardiology, gastroenterology and other fields of medicine. The ability to view subsurface structures with high resolution (2-15 μm) through small-diameter fiber-optic probes makes OCT especially useful for minimally invasive imaging of internal tissues and organs. The latest generation of OCT systems can generate OCT images up to 100 frames per second, making it possible to image coronary arteries in the beating heart artery within a few seconds. OCT can be implemented in both the time domain (TD-OCT) and the frequency domain (Fourier domain OCT or optical frequency domain imaging, OFDI).
OCT imaging of portions of a patient's body provides useful tool for doctors to determine the best type and course of treatment. For example, imaging of coronary arteries by intravascular OCT may reveal the location of a narrowing or stenosis, the presence of vulnerable plaques, and the type of atherosclerotic plaque. This information helps cardiologists to choose which treatment would best serve the patient—drug therapy (e.g., cholesterol-lowering medication), a catheter-based therapy like angioplastry and stenting, or an invasive surgical procedure like coronary bypass surgery. In addition to its applications in clinical medicine, OCT is also very useful for drug development in animal and clinical trials.
A stent is a tube-like structure that can be inserted into a vessel to expand the vessel to counteract a stenotic condition that constricts blood flow. Stents typically are made of a metal or a polymer scaffold that can be deployed to the site of a stenosis via a catheter. During percutaneous transluminal coronary angioplasty (PTCA), a factory-installed stent is usually delivered to the stenotic site through a catheter via a guide wire, and expanded using a balloon to a preset pressure to enlarge the lumen of a stenosed vessel. The first stents employed in cardiovascular medicine were made of metal without a coating, i.e., bare-metal stents (BMS). Later, to reduce the probability of restenosis, drug-eluting stents (DES) were developed on which a polymer coating containing a growth-inhibiting drug was added.
There are several factors that influence the patient outcome of deploying stents during a PTCA procedure. During PTCA, the stents should be expanded to the right diameter that corresponds to that of adjacent healthy vessel segments. Stent overexpansion may cause extensive damage to the vessel, making it prone to dissection, disarticulation, and intra-mural hemorrhage. Stent under expansion may inadequately expand the vessel to restore normal flow. If the stent struts fail to contact the vessel wall (a condition called stent malapposition), the risk of thrombosis may increase. After PTCA and stenting, the stent surface usually will be covered by a layer of endothelial cells as a result of a process called re-endothelization. Re-endothelization may be interrupted by diseases or drugs such as those used in DES. Although anticoagulant drugs are frequently prescribed for a period of 6 months to one year after the implantation of a stent, there is a the risk of a late-thrombotic event if administration of the drugs is stopped before the stent components or struts are re-endothelized completely. On the other hand, the inflammatory response of the vessel to the stent may induce excessive tissue proliferation and restenosis, possibly narrowing and closing the newly opened vessel.